Optical fiber communication is continuously experiencing the pressure of fast growing data traffic. To target the threatening capacity crunch, the communications industry is forced to upgrade and extend the existing networks. A promising enhancement possibility is the use of advanced modulation formats.
Whereas conventional optical transmission systems employ binary signalling, advanced systems rely upon high-order constellations and Polarization Division Multiplexing (PDM). These techniques improve the spectral efficiency thereby supporting the transmission of higher data rates within the same bandwidth occupied by traditional On-Off Keying (OOK) channels.
Single-carrier and Orthogonal Frequency-Division Multiplexing (OFDM) systems are well known conventional techniques. Although single-carrier PDM Quaternary Phase-Shift Keying (QPSK) is emerging as the dominant transmission scheme for the first generation of 100G systems, a clear solution for the next generations has not prevailed yet.
Generally, for the advanced modulation formats coherent receivers replace traditional direct detection receivers. By implementing a linear mapping of the optical signal into the electrical domain, coherent detection enables efficient compensation of the transmission impairments by means of digital signal processing.
Differently from On-Off Keying (OOK), which conveys information using light intensity, the new modulation formats use the phase and the polarization of the transmitted signal to encode the data. This makes them sensitive to rotations of the carrier phase and of the channel polarization. The receiver must be able to distinguish between phase and polarization modulation and unintended changes induced by the dynamic nature of the transmission channel (including transmit and receive equipment). Therefore, a coherent receiver is required to compensate not only impairments as Chromatic Dispersion (CD) and Polarization Mode Dispersion (PMD), which are relevant for a direct detector as well, but also carrier frequency offset, carrier phase noise and the dynamic variations of the transfer function of the channel. It becomes therefore increasingly difficult, especially in communications systems with high data rate, to implement sophisticated compensation algorithms because of the limited speed of digital electronics.
Coherent receivers employ digital signal processing to estimate and correct all relevant transmission impairments, including carrier frequency offset, carrier phase noise, and polarization changes.
Conventional receivers for single-carrier modulation formats are described in C. R. S. Fludger, et al., “Coherent Equalization and POLMUX-RZ-DQPSK for Robust 100-GE Transmission”, IEEE/OSA Journal of Lightwave Technology, vol. 26, no. 1, January 2008 or in M. Kuschnerov, et al., “DSP for Coherent Single-Carrier Receivers”, IEEE/OSA Journal of Lightwave Technology, vol. 27, no. 16, August 2009. A coherent OFDM receiver is demonstrated in S. L. Jansen, et al., “Long-Haul Transmission of 16×52.5 Gbits/s Polarization-Division-Multiplexed OFDM Enabled by MIMO Processing”, OSA J. Opt. Networking, vol. 7, no. 2, February 2008. Conventional receivers use either data-aided or blind algorithms to track phase noise and polarization changes.
For single-carrier PDM-QPSK transmission a conventional blind feed-forward carrier recovery can compensate at 1 dB excess penalty phase noise with a maximal combined normalized linewidth of:τsΔν≈1.6·10−3  (1)where τs is the symbol period and Δν is the sum of the full-width half-maximum linewidths of the transmit and receive local oscillator lasers (see, for example, M. G. Taylor, “Phase Estimation Methods for Optical Coherent Detection Using Digital Signal Processing”, IEEE/OSA Journal of Lightwave Technology, vol. 27, no. 7, April 2009). For a PDM-QPSK system carrying 112 Gbit/s this results into a maximal combined linewidth of 44.8 MHz at 1 dB penalty.
For a conventional coherent OFDM system (see, for example, S. L. Jansen, et al., “Coherent Optical 25.8-Gb/s OFDM Transmission Over 4160-km SSMF”, IEEE/OSA Journal of Lightwave Technology, vol. 26, no. 1, January 2008) the use of a low-power RF pilot tone allows transmitting 12.5 Gb/s using lasers with 5 MHz linewidth at 1 dB penalty, which is comparable to the single-carrier result, if the ratio between the bit rates is taken into account.
Both in single-carrier and OFDM systems, polarization demultiplexing can track polarization change rates ranging from a few tens to a few hundreds of krad/s (depending on the implementation).
As a result, both classes of systems are well able to compensate the phase noise generated by conventional Distributed Feed-Back (DFB) lasers and cope with mechanically induced polarization changes.
However, optical transmission systems, especially Wavelength-Division Multiplexing (WDM) long haul systems, are often operated in the nonlinear regime where nonlinear fiber effects induce fast phase and polarization changes. These depend on the signalling rate of the transmitted channels and have a broad spectrum extending up to the GHz region. Obviously such change rates exceed the tracking capability of the conventional algorithms described above and therefore cause residual phase and/or polarization misalignment, resulting into performance degradation.
The obvious solution of reducing the launch power and operating the system in the linear regime is unsatisfactory, because it implies a reduction of the regenerator-free reach.
Recently, compensation of nonlinear impairments has drawn some interest in the scientific community. Both electronic pre-compensation (see, for example, K. Roberts, et al., “Electronic Precompensation of Optical Nonlinearity”, IEEE Photon. Technol. Lett., vol. 18, pp. 403-405, 2006) and receiver-side coherent detection with subsequent digital signal processing (see, for example, G. Goldfarb, et al., “Experimental Demonstration of Distributed Impairment Compensation for High-Spectral Efficiency Transmission”, in Proc. Coherent Optical Technologies and Applications, Boston, Mass., p. CWB3, 2008) have been investigated.
Single channel effects, as Self Phase Modulation (SPM), can be compensated by nonlinear means on a per-channel basis. The compensation of multi-channel effects, as Cross Phase Modulation (XPM), requires the simultaneous knowledge of all affected channels. In line of principle, this could be achieved by interconnecting the relevant transponders, but, depending on network topology and wavelength routing, the interacting channels might not originate or terminate at the same site. This poses some fundamental limitations to the electronic mitigation of multi-channel effects. Equalization of single-channel effects is certainly more feasible, but also in this field the research is at an initial stage and the required implementation effort is enormous.
Polarization-time modulation for coherent optical receivers has been already investigated (see, for example, S. Mumtaz, et al., “Space-Time Codes for Optical Fiber Communication with Polarization Multiplexing”, IEEE International Conference on Communications (ICC), May 23-27, 2010). However only non-differential polarization-time codes have been taken into consideration, which unfortunately require channel knowledge at the receiver, and introduce polarization-time diversity to contrast channel imperfections, usually Polarization Dependent Loss (PDL).
The problem to be solved is to overcome the disadvantages stated above and in particular to provide coherent receiver which can compensate not only impairments as Chromatic Dispersion (CD) and Polarization Mode Dispersion (PMD), but also carrier frequency offset, carrier phase noise and the dynamic variations of the transfer function of the channel.